Superabsorbent polymer

ABSTRACT

The present invention provides a polymer derived from a cellulosic, lignocellulosic, or polysaccharide material having superabsorbent properties. Methods for making the polymer and personal care absorbent products that incorporated the polymer are also provided.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.09/939,182, filed Aug. 24, 2001, now U.S. Pat. No. 6,500,947, thebenefit of the priority of the filing date of which is hereby claimedunder 35 U.S.C. § 120. U.S. patent application Ser. No. 09/939,182 isincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a modified cellulosic, lignocelluloseor polysaccharide having superabsorbent properties and, moreparticularly, to a sulfated cellulosic, lignocellulose orpolysaccharide.

BACKGROUND OF THE INVENTION

Personal care absorbent products, such as infant diapers, adultincontinent pads, and feminine care products, typically contain anabsorbent core that includes superabsorbent polymer particlesdistributed within a fibrous matrix. Superabsorbents arewater-swellable, generally water-insoluble absorbent materials having ahigh absorbent capacity for body fluids. The superabsorbent polymers(SAP's) in common use are mostly derived from acrylic acid, which isitself derived from oil, a non-renewable raw material. Acrylic acidpolymers and SAP's are generally recognized as not being biodegradable.Despite their wide use, some segments of the absorbent products marketare concerned about the use of non-renewable oil derived materials andtheir non-biodegradable nature. Acrylic acid based polymers alsocomprise a meaningful portion of the cost structure of diapers andincontinent pads. Users of SAP are interested in lower cost SAP's. Thehigh cost derives in part from the cost structure for the manufacture ofacrylic acid which, in turn, depends upon the fluctuating price of oil.Also, when diapers are discarded after use they normally containconsiderably less than their maximum or theoretical content of bodyfluids. In other words, in terms of their fluid holding capacity, theyare “over-designed”. This “over-design” constitutes an inefficiency inthe use of SAP. The inefficiency results in part from the fact thatSAP's are designed to have high gel strength (as demonstrated by highabsorbency under load or AUL). The high gel strength (upon swelling) ofcurrently used SAP particles helps them to retain a lot of void spacebetween particles, which is helpful for rapid fluid uptake. However,this high “void volume” simultaneously results in there being a lot ofinterstitial (between particle) liquid in the product in the saturatedstate. When there is a lot of interstitial liquid the “rewet” value or“wet feeling” of an absorbent product is compromised.

In personal care absorbent products, U.S. southern pine fluff pulp iscommonly used in conjunction with the SAP. This fluff is recognizedworldwide as the preferred fiber for absorbent products. The preferenceis based on the fluff pulp's advantageous high fiber length (about 2.8mm) and its relative ease of processing from a wetlaid pulp sheet to anairlaid web. Fluff pulp is also made from renewable, and biodegradablecellulose pulp fibers. Compared to SAP, these fibers are inexpensive ona per mass basis but tend to be more expensive on a per unit of liquidheld basis. These fluff pulp fibers mostly absorb within the intersticesbetween fibers. For this reason, a fibrous matrix readily releasesacquired liquid on application of pressure. The tendency to releaseacquired liquid can result in significant skin wetness during use of anabsorbent product that includes a core formed exclusively from cellosicfibers. Such products also tend to leak acquired liquid because liquidis not effectively retained in such a fibrous absorbent core.

A need therefore exists for a superabsorbent material that issimultaneously made from a biodegradable renewable resource likecellulose or other lignocellulosic or polysaccharide, that isinexpensive, and that has a low void volume when saturated. In this waythe superabsorber can be used in absorbent product designs that areefficient such that they can be used closer to their theoreticalcapacity without feeling wet to the wearer. These and other objects areaccomplished by the invention set forth below.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a superabsorbent polymer.The polymer of the invention is a modified cellulose, modifiedlignocellulose, or modified polysaccharide having superabsorbentproperties. The modified polymer is a sulfated polymer. In oneembodiment the polymer is a sulfated cellulose; in another embodiment, asulfated lignocellulose; and in another embodiment, a sulfatedpolysaccharide. The polymer of the invention is a water-swellable,water-insoluble polymer having a high liquid absorption capacity and alow free liquid value. In one embodiment, the polymer has a liquidabsorption capacity greater than about 20 g/g. In another embodiment,the polymer has a free liquid value of less than about 40 percent byweight based on the total amount of liquid absorbed. In a furtherembodiment, the polymer has a free liquid value of less than about 30percent by weight based on the total amount of liquid absorbed. In oneembodiment, the modified polymer is a crosslinked polymer.

In other aspects of the invention, methods for making the superabsorbentpolymer and absorbent products that incorporate the superabsorbentpolymer are provided.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a cross sectional view of an absorbent construct incorporatinga representative polymer of the invention and having an acquisitionlayer;

FIG. 2 is a cross sectional view of an absorbent construct incorporatinga representative polymer of the invention and having acquisition anddistribution layers; and

FIGS. 3A-C are cross sectional views of absorbent articles incorporatinga composite including representative polymer of the invention and theabsorbent constructs illustrated in FIGS. 1 and 2, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In one aspect, the present invention provides a superabsorbent polymer.The polymer of the invention is a modified cellulose, modifiedlignocellulose, or modified polysaccharide having superabsorbentproperties. The modified polymer is a sulfated polymer. In oneembodiment the polymer is a sulfated cellulose; in another embodiment, asulfated lignocellulose; and in another embodiment, a sulfatedpolysaccharide.

The polymer of the invention is a water-swellable, water-insolublepolymer having a high liquid absorption capacity. Water swellability isimparted to the polymer through sulfation. The polymer has a degree ofsulfate group substitution effective to provide advantageous waterswellability. The polymer has a liquid absorption capacity that isincreased compared to unmodified fluff pulp fibers. In one embodiment,the polymer has a liquid absorption capacity greater than about 20 g/g.

Cellulosic fibers suitable for use in forming the modified cellulosepolymer of the present invention are substantially water-insoluble andnot highly water-swellable. After sulfation in accordance with thepresent invention, the resulting modified polymer has the desiredabsorbency characteristics, is water-swellable and water-insoluble.

The polymer of the invention is substantially insoluble in water. Asused herein, a material will be considered to be water-soluble when itsubstantially dissolves in excess water to form a solution, losing itsform and becoming essentially evenly disbursed throughout a watersolution.

The polymer of the invention is a water-swellable, water-insolublepolymer. As used herein, the term “water-swellable, water-insolublepolymer” refers to a polymer that, when exposed to an excess of anaqueous medium (e.g., bodily fluids such as urine or blood, water,synthetic urine, or 1 weight percent solution of sodium chloride inwater), swells to an equilibrium volume but does not dissolve intosolution.

In addition to having a high liquid absorption capacity, the polymer ofthe invention has a low free liquid value. As used herein, the term“free liquid value” refers to the amount of liquid that is present in amaterial that is free and can be readily expelled from the material. Thefree liquid value relates to the structure of a material. Materialshaving high void volumes (e.g., large interstitial spaces) will haverelatively high free liquid values, and materials having lesser voidvolumes and smaller interstitial spaces will have relatively low freeliquid values. Materials that include conventional superabsorbentpolymers, which are substantially spherical in shape and have beendesigned to have high gel strength, have relatively large void volumesand therefore retain relatively greater amounts of free liquid. Suchmaterials are in contrast to materials that include the polymer of theinvention that are characterized as having small void volumes and retainrelatively lesser amounts of free liquid. The difference in free liquidreflects the structure of the two absorbent materials. Packing ofconventional substantially spherical polymers results in interstitialspaces capable of retaining relatively greater amount of free liquid.The polymer of the invention packs without forming as great interstitialspaces and, consequently, relatively less free liquid is retained.

As used herein, the term “free liquid value” is defined as thepercentage of liquid absorbed by a material that can be subsequentlyexpelled from the material on centrifugation. Free liquid value isdetermined by allowing a material to swell in the liquid absorbing itsmaximum amount of that liquid, followed by centrifuging theliquid-swelled material to expel free liquid. The difference in weightof the material after initial swelling and the weight of the materialafter centrifugation to remove free liquid divided by the materialweight after initial swelling is the percentage of free liquid absorbedby the material and is the free liquid value. Free liquid value isdetermined as described in Example 3. In one embodiment, the polymer ofthe invention has a free liquid value of less than about 40 percent byweight based on the total amount of liquid absorbed. In anotherembodiment, the polymer has a free liquid value of less than about 30percent by weight based on the total amount of liquid absorbed. Freeliquid values for representative polymers of the invention are describedin Table 4 in Example 4.

In one embodiment, cellulosic fibers are a starting material forpreparing the polymer of the invention. Although available from othersources, suitable cellulosic fibers are derived primarily from woodpulp. Suitable wood pulp fibers for use with the invention can beobtained from well-known chemical processes such as the kraft andsulfite processes, with or without subsequent bleaching. Pulp fibers canalso be processed by thermomechanical, chemithermomechanical methods, orcombinations thereof. Caustic extractive pulp such as TRUCELL,commercially available from Weyerhaeuser Company, is also a suitablewood pulp fiber. A preferred pulp fiber is produced by chemical methods.Ground wood fibers, recycled or secondary wood pulp fibers, and bleachedand unbleached wood pulp fibers can be used. Softwoods and hardwoods canbe used. Details of the selection of wood pulp fibers are well-known tothose skilled in the art. These fibers are commercially available from anumber of companies, including Weyerhaeuser Company, the assignee of thepresent invention. For example, suitable cellulosic fibers produced fromsouthern pine that are usable with the present invention are availablefrom Weyerhaeuser Company under the designations CF416, NF405, PL416,FR416, and NB416. In one embodiment, the cellulosic fiber useful inmaking the polymer of the invention is a southern pine fibercommercially available from Weyerhaeuser Company under the designationNB416. In other embodiments, the cellulosic fiber can be selected fromamong a northern softwood fiber, a eucalyptus fiber, a rye grass fiber,and a cotton fiber.

Cellulosic fibers having a wide range of degree of polymerization aresuitable for forming the polymer of the invention. In one embodiment,the cellulosic fiber has a relatively high degree of polymerization,greater than about 1000, and in another embodiment, about 1500.

In one embodiment, the polymer of the invention is a sulfated cellulosepolymer. As used herein, “sulfated cellulose” refers to cellulose thathave been sulfated by reaction with a sulfating agent. It will beappreciated that the term “sulfated cellulose” includes free acid andsalt forms of sulfated cellulose. Suitable metal salts include sodium,potassium, and lithium salt, among others. A sulfated cellulose polymercan be produced by reacting a sulfating agent with a hydroxyl group ofthe cellulose to provide a cellulose sulfate ester (i.e., acarbon-to-oxygen-to-sulfur ester). The sulfated cellulose polymer formedin accordance with the present invention differs from othersulfur-containing cellulosic compounds in which the sulfur atom isattached directly to a carbon atom on the cellulose chain as, forexample, in the case of sulfonated cellulose; or cellulosic compounds inwhich the sulfate sulfur atom is attached indirectly to a carbon atom onthe cellulose chain as, for example, in the case of cellulose alkylsulfonates.

The sulfated cellulose polymer of the invention can be characterized ashaving an average degree of sulfate group substitution of from about 0.1to about 2.0. In one embodiment, the sulfated cellulose has an averagedegree of sulfate group substitution of from about 0.2 to about 1.0. Inanother embodiment, the sulfated cellulose has an average degree ofsulfate group substitution of from about 0.3 to about 0.5. As usedherein, the “average degree of sulfate group substitution” refers to theaverage number of moles of sulfate groups per mole of glucose unit inthe polymer. It will be appreciated that the polymers formed inaccordance with the present invention include a distribution of sulfatemodified polymers having an average degree of sulfate substitution asnoted above.

The polymer of the invention is a sulfated polymer. In one embodiment,the polymer is derived from cellulosic fibers. Sulfated cellulosicfibers can be made by reacting cellulosic fibers (e.g., cellulosicfibers that are crosslinked or noncrosslinked) with a sulfating agent.Suitable sulfating agents include concentrated sulfuric acid (95-98%),fuming sulfuric acid (oleum), sulfur trioxide and related complexesincluding sulfur trioxide/dimethylformamide and sulfur trioxide/pyridinecomplexes, and chlorosulfonic acid, among others. In one embodiment, thesulfating agent is concentrated sulfuric acid.

The sulfating agent is preferably applied to the fibers as a solution inan organic solvent. Suitable organic solvents include alcohols,pyridine, dimethylformamide, acetic acid including glacial acetic acid,and dioxane. In one embodiment, the organic solvent is an alcohol havingup to about 6 carbon atoms. Suitable alcohols include methanol, ethanol,n-propanol, isopropanol, n-butanol, isobutanol, s-butanol, pentanols,and hexanols. In one embodiment, the alcohol is selected from amongisopropanol and isobutanol.

The molar ratio of sulfuric acid to alcohol in the solution can bevaried from about 1:1 to about 4:1. In one embodiment, the molar ratioof sulfuric acid to alcohol is about 2.4:1, for example, an 80:20(weight/weight) solution of sulfuric acid in isopropanol. The weightratio of sulfuric acid to cellulosic fibers in the sulfation reactioncan be varied from about 5:1 to about 30:1. At low sulfuric acid ratiosthe reaction is slow and incomplete and at high sulfuric acid ratiossignificant cellulose polymer degradation can occur. In one embodiment,the weight ratio of sulfuric acid to pulp fiber is from about 10:1 toabout 25:1. In another embodiment, the weight ratio of sulfuric acid topulp fiber is about 24:1.

Highly acidic aqueous environments readily degrade cellulose fibers. Ithas been reported that concentrated sulfuric acid cannot be used toprepare sulfated cellulose because treating cellulose with sulfuric acidresults in a soluble product formed from acid hydrolysis of thecellulose backbone by the sulfuric acid. See, WO 96/15137. However, awater-soluble cellulose sulfate has been reportedly prepared from anactivated cellulose (20 to 30% water) by direct action of aqueoussulfuric acid or sulfuric acid dissolved in a volatile organic solventsuch as toluene, carbon tetrachloride, or a lower alkanol. “CelluloseChemistry and Its Applications”, Ed. T. P. Nevell and S. H. Zeronian,Halstead Press, John Wiley and Sons, 1985, page 350.

Despite the well-known degradation of cellulose in aqueous acidicsolutions, the present invention provides methods for making sulfatedcellulose polymers without significant cellulose hydrolysis. In themethods of the invention, cellulose degradation (i.e., degree ofpolymerization reduction) is substantially avoided by treating cellulosefibers with a sulfating agent in a nonaqueous environment and/or at lowtemperature (e.g., at or below about 4° C.). To further protect againstcellulose degradation (e.g., hydrolysis), a stabilizing agent ordehydrating agent to absorb water, including water formed during thesulfation reaction, can be added to the sulfating reaction mixture.Suitable stabilizing or dehydrating agents include, for example, sulfurtrioxide, magnesium sulfate, acetic anhydride, and molecular sieves. Inone embodiment, cellulosic fibers are reacted with the sulfating agentat a temperature of about 4° C. and both the cellulosic fibers and thesulfating agent are cooled to about 4° C. prior to reaction. In anotherembodiment, cellulosic fibers, including cooled fibers, are reacted withthe sulfating agent in the presence of a dehydrating agent.

Depending upon the extent of sulfation desired, the fibers and sulfatingagent are reacted for a period of time of from about 10 to about 300minutes. Following this reaction period and prior to neutralizing theresulting sulfated fibers, the sulfated fibers are separated from excesssulfating agent. In one embodiment, the sulfated fibers are washed withan alcohol prior to neutralization.

Prior to further processing the sulfated cellulosic fibers (e.g.,crosslinking) to provide the polymer of the invention, the fibers can beat least partially neutralized with a neutralizing agent. Theneutralizing agent is suitably soluble in the sulfation solvent. In oneembodiment, the neutralizing agent is a base such as, for example, analkaline base (e.g., lithium, potassium, sodium or calcium hydroxide;lithium, potassium, or sodium acetate). Alternatively, the neutralizingagent can include a multivalent metal salt. Suitable metal salts includecerium, magnesium, calcium, zirconium, and aluminum salts such asammonium cerium nitrate, magnesium sulfate, magnesium chloride, calciumchloride, zirconium chloride, aluminum chloride, and aluminum sulfate,among others. The use of multivalent metal salts as neutralizing agentsalso offers the advantage of intrafiber crosslinking. Thus, through theuse of a multivalent metal salt, the sulfated cellulosic can bepartially neutralized and partially crosslinked. Fibers so treated canbe further crosslinked with other crosslinking agents including thosedescribed above.

The extent of fiber sulfation is dependent on a number of reactionconditions including reaction time. For example, in a series ofrepresentative sulfation reactions, a 25 minute reaction time provided afiber that included about 3.8 percent by weight sulfur; a 35 minutereaction time provided a fiber that included about 4.9 percent by weightsulfur; and a 45 minute reaction time provided a fiber that includedabout 6.4 percent by weight sulfur. However, in these experiments, theextended sulfation reaction time had an adverse effect on fiber length(i.e., cellulose hydrolysis occurred under the prolonged reactionconditions). In viscosity experiments, the sulfated fibers produced bythe 25 and 35 minute reaction conditions provide cellulose solutionsclassified as having a Gardner-Holt bubble tube H viscosity (i.e., about200 Centistokes), while the sulfated fibers produced by the 45 minutereaction provided cellulose solutions classified as having C viscosity(i.e., about 85 Centistokes). The results indicate that at extendedreaction times, significant fiber degradation can occur. The absorbentcapacity of modified fibers prepared from sulfated fibers is describedin Example 3.

Representative methods for preparing a sulfated cellulose polymer aredescribed in Examples 1 and 4.

In one embodiment, the modified polymer is a crosslinked polymer.Crosslinked cellulosic fibers and methods for their preparation aredisclosed in U.S. Pat. Nos. 5,437,418 and 5,225,047 issued to Graef etal., expressly incorporated herein by reference.

Crosslinked cellulose fibers can be prepared by treating fibers with acrosslinking agent. Suitable crosslinking agents useful in producing thepolymer are generally soluble in water and/or alcohol. Suitablecellulosic fiber crosslinking agents include aldehyde, dialdehyde, andrelated derivatives (e.g., formaldehyde, glyoxal, glutaraldehyde,glyceraldehyde), and urea-based formaldehyde addition products (e.g.,N-methylol compounds). See, for example, U.S. Pat. Nos. 3,224,926;3,241,533; 3,932,209; 4,035,147; 3,756,913; 4,689,118; 4,822,453; U.S.Pat. No. 3,440,135, issued to Chung; U.S. Pat. No. 4,935,022, issued toLash et al.; U.S. Pat. No. 4,889,595, issued to Herron et al.; U.S. Pat.No. 3,819,470, issued to Shaw et al.; U.S. Pat. No. 3,658,613, issued toSteiger et al.; and U.S. Pat. No. 4,853,086, issued to Graef et al., allof which are expressly incorporated herein by reference in theirentirety. Cellulosic fibers can also be crosslinked by carboxylic acidcrosslinking agents including polycarboxylic acids. U.S. Pat. Nos.5,137,537; 5,183,707; and 5,190,563, describe the use of C2-C9polycarboxylic acids that contain at least three carboxyl groups (e.g.,citric acid and oxydisuccinic acid) as crosslinking agents.

Suitable urea-based crosslinking agents include methylolated ureas,methylolated cyclic ureas, methylolated lower alkyl substituted cyclicureas, methylolated dihydroxy cyclic ureas, dihydroxy cyclic ureas, andlower alkyl substituted cyclic ureas. Specific preferred urea-basedcrosslinking agents include dimethylol urea (DMU,bis[N-hydroxymethyl]urea), dimethylolethylene urea (DMEU,1,3-dihydroxymethyl-2-imidazolidinone), dimethyloldihydroxyethylene urea(DMDHEU, 1,3-dihydroxymethyl-4,5-dihydroxy-2-imidazolidinone),dimethylolpropylene urea (DMPU), dimethylolhydantoin (DMH),dimethyldihydroxy urea (DMDHU), dihydroxyethylene urea (DHEU,4,5-dihydroxy-2-imidazolidinone), and dimethyldihydroxyethylene urea(DMeDHEU, 4,5-dihydroxy-1,3-dimethyl-2-imidazolidinone).

Suitable polycarboxylic acid crosslinking agents include citric acid,tartaric acid, malic acid, succinic acid, glutaric acid, citraconicacid, itaconic acid, tartrate monosuccinic acid, maleic acid,1,2,3-propane tricarboxylic acid, 1,2,3,4-butanetetracarboxylic acid,all-cis-cyclopentane tetracarboxylic acid, tetrahydrofurantetracarboxylic acid, 1,2,4,5-benzenetetracarboxylic acid, andbenzenehexacarboxylic acid. Other polycarboxylic acids crosslinkingagents include polymeric polycarboxylic acids such as poly(acrylicacid), poly(methacrylic acid), poly(maleic acid),poly(methylvinylether-co-maleate) copolymer,poly(methylvinylether-co-itaconate) copolymer, copolymers of acrylicacid, and copolymers of maleic acid. The use of polymeric polycarboxylicacid crosslinking agents such as polyacrylic acid polymers, polymaleicacid polymers, copolymers of acrylic acid, and copolymers of maleic acidis described in U.S. Pat. No. 5,998,511, assigned to WeyerhaeuserCompany and expressly incorporated herein by reference in its entirety.

Other suitable crosslinking agents include diepoxides such as, forexample, vinylcyclohexene dioxide, butadiene dioxide, and diglycidylether; sulfones such as, for example, divinyl sulfone,bis(2-hydroxyethyl)sulfone, bis(2-chloroethyl)sulfone, and disodiumtris(β-sulfatoethyl)sulfonium inner salt; and diisocyanates.

Mixtures and/or blends of crosslinking agents can also be used.

For embodiments of the polymer that are crosslinked with a crosslinkingagent, a catalyst can be used to accelerate the crosslinking reaction.Suitable catalysts include acidic salts, such as ammonium chloride,ammonium sulfate, aluminum chloride, magnesium chloride, and alkalimetal salts of phosphorous-containing acids.

The amount of crosslinking agent applied to the polymer is suitably theamount necessary to render the polymer substantially insoluble in water.The amount of crosslinking agent applied to the polymer will depend onthe particular crosslinking agent and is suitably in the range of fromabout 0.01 to about 8.0 percent by weight based on the total weight ofpolymer. In one embodiment, the amount of crosslinking agent applied tothe polymer is in the range from about 0.20 to about 5.0 percent byweight based on the total weight of polymer.

The polymer of the invention has a liquid absorbent capacity of at leastabout 20 g/g as measured by the centrifuge capacity test described inExample 3. In one embodiment, the modified polymer has a capacity of atleast about 10 g/g. In another embodiment, the polymer has a capacity ofat least about 15 g/g, and in a further embodiment, the polymer has acapacity of at least about 20 g/g. The absorbent capacity ofrepresentative polymers formed in accordance with the present inventionis described in Example 3.

In another aspect of the invention, a method for manufacturing asuperabsorbent polymer is provided. In one embodiment of the method,cellulosic fibers are sulfated by reacting the fibers with a sulfatingagent.

For effective sulfation, cellulosic fibers, including dried fibers, canbe swelled prior to sulfation using a swelling agent. Suitable swellingagents include, for example, water, glacial acetic acid, aceticanhydride, zinc chloride, sulfuric acid, sulfur trioxide, and ammonia.The fibers can be swelled by mixing the fibers with the swelling agentfollowed by removing excess swelling agent prior to reacting the fiberswith the sulfating agent.

Thus, in another embodiment, the present invention provides a method formaking a superabsorbent polymer that includes the steps of swellingcellulosic fibers, including dry fibers, with a swelling agent;separating excess swelling agent from the swelled fibers; reacting theswelled fibers with a sulfating agent; separating excess sulfating agentfrom the fibers; and at least partially neutralizing the sulfated fibersto provide sulfated cellulosic fibers.

The polymer of the invention can be provided in particle form bydissolving the sulfated polymer formed in the sulfation step in anaqueous medium followed by precipitation of the polymer from the medium.Precipitation of the polymer from the aqueous medium can be achieved bydiluting the aqueous medium containing the polymer with a misciblesolvent in which the polymer has a low solubility. The polymer particlesprepared by this method is referred to as “regenerated” polymer particlebecause the polymer particle is produced by precipitation from a polymersolution. Precipitation of the polymer can be achieved using anon-aqueous material, such as alcohol (e.g. isopropanol) or a salt. Amethod for preparing a regenerated polymer particle is described inExample 4. Alternatively, the polymer may also be recovered (or“regenerated”) from solution simply by drying off the water from theaqueous solution.

As noted above, in one embodiment, the polymer of the invention is asulfated and crosslinked polymer. Accordingly, sulfated polymersprepared as described above can be further crosslinked with acrosslinking agent to provide the superabsorbent polymer. Arepresentative method for making a sulfated and crosslinkedsuperabsorbent polymer is described in Example 2.

The polymers of the invention can be incorporated into a personal careabsorbent product. The polymers can be formed into a composite forincorporation into a personal care absorbent product. Composites can beformed from the polymer alone or by combining the polymer with othermaterials, including fibrous materials, binder materials, otherabsorbent materials, and other materials commonly employed in personalcare absorbent products. Suitable fibrous materials include syntheticfibers, such as polyester, polypropylene, and bicomponent bindingfibers; and cellulosic fibers, such as fluff pulp fibers, crosslinkedcellulosic fibers, cotton fibers, and CTMP fibers. Suitable absorbentmaterials include natural absorbents, such as sphagnum moss, andsynthetic superabsorbents, such as polyacrylates (e.g., SAPs).

Absorbent composites derived from or that include the polymers of theinvention can be advantageously incorporated into a variety of absorbentarticles such as diapers including disposable diapers and trainingpants; feminine care products including sanitary napkins, and pantliners; adult incontinence products; toweling; surgical and dentalsponges; bandages; food tray pads; and the like. Thus, in anotheraspect, the present invention provides absorbent composites, constructs,and absorbent articles that include the polymer.

The superabsorbent polymer of the invention can be incorporated as anabsorbent core or storage layer into a personal care absorbent productsuch as a diaper. The composite can be used alone or combined with oneor more other layers, such as acquisition and/or distribution layers, toprovide useful absorbent constructs.

Representative absorbent constructs incorporating an absorbent compositethat includes a polymer of the invention are shown in FIGS. 1 and 2.Referring to FIG. 1, construct 100 includes composite 10 (i.e., acomposite that includes a polymer of the invention) employed as astorage layer in combination with an upper acquisition layer 20.

In addition to the construct noted above that includes the combinationof absorbent composite and acquisition layer, further constructs caninclude a distribution layer intermediate the acquisition layer andcomposite. FIG. 2 illustrates construct 110 having intermediate layer 30(e.g., distribution layer) interposed between acquisition layer 20 andcomposite 10.

Composite 10 and constructs 100 and 110 can be incorporated intoabsorbent articles. Generally, absorbent articles 200, 210, and 220shown in FIGS. 3A-C, include liquid pervious facing sheet 22, liquidimpervious backing sheet 24, and a composite 10, construct 100,construct 110, respectively. In such absorbent articles, the facingsheet can be joined to the backing sheet.

It will be appreciated that other absorbent products can be designedincorporating the polymer of the invention and composites that includethe polymer.

The following examples are provided for the purposes of illustrating,not limiting, the present invention.

EXAMPLE 1 The Preparation of Sulfated Cellulosic Fibers

In this example, a representative method for forming sulfated cellulosicfibers is described.

Prior to sulfation, the pulp was activated with acetic acid. Ten gramsof fiberized bleached kraft southern yellow pine fluff pulp (NB416,Weyerhaeuser Company, Federal Way, Wash.) that had been oven dried at105° C. was disbursed in 600 mL of glacial acetic acid. The pulp/acidslurry was then placed in a vacuum chamber and the air was evacuated.The slurry was allowed to stand under vacuum for 30 minutes after whichtime the chamber was repressurized to atmospheric pressure. The slurrywas then allowed to stand at ambient conditions for 45 minutes beforebeing resubjected to a vacuum for an additional 30 minutes. After thesecond application of a vacuum the slurry was again allowed to stand for45 minutes at atmospheric pressure. The slurry was then poured into aBuchner funnel where the pulp was collected and pressed until the weightof the residual acetic acid was equal to twice the weight of the ovendry pulp (i.e., total weight of the collected pulp was 30 g.) Thecollected pulp was placed inside a plastic bag and cooled to −10° C. ina freezer.

The sulfation liquor was prepared by mixing 240 g concentrated sulfuricacid with 60 g isopropanol and 0.226 g magnesium sulfate. The liquor wasprepared by pouring isopropanol into a beaker that was maintained at 4°C. in an ice bath. Magnesium sulfate was then added to the isopropanoland the mixture chilled to 4° C. Sulfuric acid was weighed into a beakerand separately chilled to 9° C. before being slowly mixed into theisopropanol and magnesium sulfate mixture. The resulting sulfatingliquor was then allowed to cool to 4° C.

The cooled acetic acid activated pulp (−10° C.) was stirred into thecooled sulfation liquor (4° C.). The resulting slurry of pulp andsulfation liquor was allowed to react for 35 minutes with constantstirring. After 35 minutes the pulp/sulfation liquor slurry was pouredinto a Buchner funnel and the sulfated pulp was collected and washedover a vacuum with cooled isopropanol (−10° C.). The collected pulp wasthen slurried with cooled isopropanol (−10° C.) in a Waring blender andpoured back into the Buchner funnel where the pulp was again washed withcooled isopropanol (−10° C.).

The nature and quality of the modified fiber formed in accordance withthe invention can depend on the washing step. First, the acid ispreferably washed from the pulp as quickly as possible to preventcontinued and/or accelerated cellulose degradation. Second, the cooltemperature of the pulp is preferably maintained to prevent cellulosedegradation. Third, the acid is preferably washed from the pulp asthoroughly as possible before neutralization to prevent the formation ofdifficult to remove inorganic salts during the neutralization step.These salts can adversely impact modified fiber absorbency.

The washed sulfated pulp was next slurried in cooled isopropanol (−10°C.) and an ethanolic sodium hydroxide solution (saturated solution) wasadded dropwise until the slurry was neutralized. The slurry was thenpoured into a Buchner funnel where the neutralized sulfated pulp waswashed with room temperature isopropanol. The neutralized sulfated pulpwas then agitated to remove any inorganic salts that may have beencrusted on the fiber surfaces after which the neutralized sulfated pulpwas again washed with isopropanol in a Buchner funnel. Finally thecollected sulfated pulp was allowed to air dry.

EXAMPLE 2 The Preparation of Representative Crosslinked, SulfatedCellulosic Fibers

In this example, a representative method for forming crosslinked,sulfated cellulosic fibers is described. Sulfated cellulosic fibersprepared as described in Example 1 were crosslinked with arepresentative crosslinking agent.

A catalyzed urea-formaldehyde system was used to crosslink the sulfatedcellulosic fibers. The catalyst included magnesium chloride and thesodium salt of dodecylbenzenesulfonic acid dissolved in 88%ethanol/water. In addition to its primary function, the catalystsolution served as a diluent for the crosslinking agent. Thecrosslinking agent was obtained by dissolving urea in 37 percent (w/w)aqueous formaldehyde. The crosslinking agent was combined with thecatalyst solution and applied to the sulfated fibers. The treated fiberswere then cured by placing in a 105° C. oven for 60 minutes.

In the experiment, varying amounts of crosslinking agents were appliedto the fibers. The amount of crosslinking agent used ranged from 1-11percent of the weight of the sulfated fibers and the amount of catalyticdiluent used was 250 percent of the weight of the sulfated fibers. Thematerials and their amounts used in preparing the catalytic diluent andcrosslinking agent solutions are shown in Table 1 below.

TABLE 1 Composition of Catalytic Diluent and Crosslinking AgentSolution. Parts Catalytic Diluent Denatured ethanol 44 Deionized water 6Magnesium chloride heptahydrate 0.214 Dodecylbenzenesulfonic acid,sodium salt 0.4 Crosslinking Agent Solution Urea 15 37% (w/w)Formaldehyde 41

EXAMPLE 3 Method for Determining Total Absorptive Capacity and Tea BagVolume Test

In this example, a method for determining total absorptive capacity andtea bag gel volume test are described. Representative modified fibers,prepared as described in Examples 1 and 2 above, with varying levels ofcrosslinking agent applied to the fibers were evaluated for absorbentcapacity by the total absorptive capacity/tea bag gel volume testdescribed below. Modified fiber absorbent capacity as a function ofcrosslinking agent applied to the fiber is summarized in Table 2 below.

The preparation of materials, test procedure, and calculations todetermine absorbent capacity were as follows.

Preparation of Materials:

1) Tea bag preparation: unroll tea bag material (Dexter #1234Theat-sealable tea bag material) and cut cross ways into 6 cm pieces.Fold lengthwise, outside-to-outside. Heatseal edges ⅛ inch with an iron(high setting), leave top end open. Trim excess from top edge to form a6 cm×6 cm bag. Prepare 3 tea bags.

2) Label edge with sample identification.

3) Weigh 0.200 g sample (nearest 0.001 g) on tared glassine and recordweight. (Weight A, see below.)

4) Fill tea bags with modified fiber sample.

5) Seal top edge of tea bag ⅛ inch with the iron.

6) Weigh and record total weight of tea bag filled with modified fibersample. Store in sealed plastic bag until ready to test (Weight B, seebelow).

Test Procedure:

1) Fill container to a depth of at least 2 inch with 1 percent by weightsaline solution.

2) Hold tea bag horizontally and distribute modified fiber sample evenlythroughout tea bag.

3) Lay tea bag on the liquid surface of the saline solution (begintiming) and allow tea bag to wet-out before submerging the tea bag(about 10 sec.).

4) Soak tea bag for 30 minutes. (NOTE: 2 hours was used in the case ofthe samples from Example 4.)

5) Remove tea bag from the saline solution with tweezers and clip to adrip rack.

6) Allow tea bag to hang for 3 minutes.

7) Carefully remove tea bag from clip and lightly touch saturated cornerof tea bag on blotter to remove excess fluid. Weigh tea bag and recordweight (i.e., drip weight) (Weight C, see below).

Calculation of Before Centrifuge Capacity (Z=g/g Capacity):Z=(C−B)/A

This calculation assumes that the liquid held by the tea bag material isa negligible and reasonably constant factor.

8) Place tea bag on wall of centrifuge by pressing top edge against thewall. Balance centrifuge by placing the tea bags around the centrifuge'scircumference.

9) Centrifuge at 2800 rpm for 75 seconds.

10) Remove tea bag from centrifuge, weigh and record tea bag centrifugedweight (Weight D, see below).

Calculation of After Centrifuge Capacity (Y=g/g Capacity):Y=(D−B)/A

This calculation assumes that the liquid held by the tea bag material isa negligible and reasonably constant factor.

Calculation of Free Liquid Value as a Percentage:Percent free liquid=((Z−Y)/Z)×100

The after centrifuge absorbent capacity (centrifuge capacity, g/g),determined as described above, as a function of sulfation reaction timeand crosslinking agent applied to the fiber for representative modifiedfibers is summarized in Table 2 below.

TABLE 2 Modified Fiber Absorbent Capacity: Crosslinking Level andSulfation Reaction Time Effect. Crosslinking Centrifuge Capacity (g/g)level (percent 25 minute 35 minute 45 minute by weight) sulfationsulfation sulfation 1.08 13.0 12.1 7.0 1.62 15.3 14.6 10.1 1.94 17.22.27 15.1 2.48 17.3 2.27 14.7 18.0 2.97 11.3 3.24 11.9 3.78 8.1 7.9 8.64.00 6.6

As shown in Table 2, to a point, absorbent capacity increases withincreasing sulfation as all the sulfated fibers have capacities that arehigher than one would expect for untreated fiber (only ˜1 g/g or so).However, at the point where sulfation results in fiber degradation,absorbent capacity decreases. The results also demonstrate thatabsorbent capacity also increases with increasing crosslinking to apoint. At higher levels of crosslinking, absorbent capacity decreases.

EXAMPLE 4 Representative Superabsorbent Polymer Particle PreparationMethod

In this example, a method for preparing a representative superabsorbentcellulosic polymer particle is described. In the method, the polymer isprepared by first sulfating cellulosic pulp followed by dissolving inwater and then precipitating the sulfated cellulosic pulp to provide asuperabsorbent cellulosic polymer particle.

Cellulosic Pulp Sulfation

Solvent Exchange and Pretreatment of the Never-Dried Pulp. Using a glassstir rod, slurry 50 g (OD basis) of never-dried FR-416 pulp (ca. 25%solids) with 2.7L of glacial acetic acid in a glass beaker. Allow theacid/pulp slurry to stand for 30 min. Drain and press the slurry overvacuum in the Buchner funnel. Using a glass stir rod, reslurry thefilter pad with 2.7L of glacial acetic acid in a glass beaker. Allow theacid/pulp slurry to stand for 30 min. Drain and press the slurry overvacuum in the Buchner funnel. Using a glass stir rod, reslurry thefilter pad with 2.7L of glacial acetic acid in a glass beaker. Allow theacid/pulp slurry to stand for 30 min. Drain the slurry over vacuum inthe Buchner funnel and press the filter pad until the weight of theresidual acetic acid is equal to about three times the oven dry weightof the pulp. The total weight of the filter pad will be about 200 g, 50g of which will be accounted for by the mass of the pulp with theremaining 150 g resulting from residual acetic acid. Shred the filterpad into a ZIPLOC bag and chill to −10° C. in the freezer.

Preparation of the Sulfation Liquor. Place a Hobart mixer and the heatexchanger into an insulated cooling chest. Pour propylene glycol intothe insulated cooling chest until all but the top 2 inches of the mixerbowl is submerged. Connect a heat exchanger to the a Julabo FPW55-SPwater-cooled ultra-low refrigerated circulator. Fill the tank of thecirculator with Baysilone Fluid M3 (a polydimethylsiloxane manufacturedby GE Bayer Silicones and distributed by Julabo as Thermal HY). Programthe desired temperature into the circulator and start the unit. In thisexample a temperature of −10° C. was used. Wait for the propylene glycolin the insulated cooling chest to reach the programmed temperature. Pour300 g of −10° C. isopropanol into the bowl of the mixer. Add 1.13 g ofmagnesium sulfate to the isopropanol. Slowly stir 1,200 g of 9° C.concentrated sulfuric acid into the isopropanol. Allow the sulfationliquor to cool to the desired reaction temperature. The ratios of thecomponents in the sulfation liquor will be: 4 parts concentratedsulfuric acid, 1 part isopropanol, and 0.004 parts magnesium sulfate.

Sulfation Reaction. Route the mixer's power supply through a rheostat.With the rheostat set at zero switch the mixer on to its lowest setting.Dial up the rheostat until the mixer blade is turning slowly. Feed theshredded −10° C. acetic acid activated FR-416 pulp into the runningmixer at a rate that will not overload the mixer, yet is not so slow asto cause the sulfation time of the first pulp in to be significantlydifferent from that of the last pulp in. Maintain the appropriatereaction temperature and allow the pulp to react for the time indicatedin Table 3 while undergoing constant stirring. The weight of thesulfation liquor will be 30 times that of the solvent exchanged,never-dried FR-416 pulp being sulfated (not including the weight of theresidual acetic acid.)

Filtration and Washing of the Sulfated Pulp. Upon completion of thesulfation reaction, pour the pulp/sulfation liquor slurry into a Buchnerfunnel and remove as much sulfation liquor from the pulp as possible.Pour 3L of −10° C. isopropanol through the filter pad to wash away whatacid could not be physically removed from the pulp. Disintegrate thefilter pad in the Waring blender and pour the resultant slurry into aBuchner funnel. Pour 3L of −10° C. isopropanol through the filter pad.

There are four important factors in the washing stage. Firstly, to avoidan excessive generation of heat via the mixing of the wash isopropanolwith an overabundance of sulfuric acid, as much sulfation liquor aspossible should be expressed from the pulp prior to washing. Secondly,the acid must be washed from the pulp as quickly as possible to preventcontinued and accelerated degradation to the cellulose. Thirdly, thetemperature of the pulp should not be allowed to rise because thecellulose may be extensively damaged. Fourthly, the acid must be washedout as thoroughly as possible before neutralization to prevent theformation of difficult to remove inorganic salts, which would ultimatelyhave a negative impact on the absorbency of the resultant polymer.

Neutralization of the Cellulose Sulfate. Using a pneumatic poweredmixer, slurry the washed cellulose sulfate with −10° C. isopropanol inthe 4L plastic beaker. Maintain stirring while adding a saturatedethanolic sodium acetate solution to the slurry dropwise with a buretteuntil a pH of 7 is attained. Drain and press the slurry over vacuum inthe Buchner funnel. Agitate the sodium cellulose sulfate in roomtemperature isopropanol with the Waring blender to remove any inorganicsalts that may have crusted on the fiber surfaces. Drain and press theslurry over vacuum in the Buchner funnel. Wash the filter pad with roomtemperature isopropanol. Shred the filter pad and allow the sodiumcellulose sulfate to air dry in a fume hood.

TABLE 3 Representative Polymer Sulfation Reaction Times. Sample NumberSulfation Reaction Time (mins) A 180 B 180 C 240

Sodium Cellulose Sulfate Polymer Precipitation

To 13 g water, add 1 gm of the neutralized cellulose sulfate sampleprepared as described above. Stir to dissolve the fiber. Stirperiodically until complete dissolution is obtained (typically 1-24hours). Add about 125 ml of isopropanol to the cellulose sulfatesolution to precipitate the polymer. Mix well with a small plasticspatula and then pull the precipitated material apart to form small(from about 5 mm to about 10 mm in largest dimension) sized, elongatedpieces of precipitate. Filter excess solvent off in a Buchner funnel.Add an additional 125 mL isopropanol to the precipitate as an additionalwash. Filter again. Dry the precipitate at 105° C. for 60 minutes in alaboratory fan oven. The free swell and centrifuge capacities weredetermined as described above in Example 3. The capacities beforecentrifuge (See Z above), capacities after centrifuge (see Y above), andFree Liquid Value (percent free liquid) for representative polymersprepared as described above are summarized in Table 4 below.

It should be noted in this Example 4, in contrast to Example 2, that nocrosslinking agent was used in preparing the superabsorbent polymerparticles.

TABLE 4 Representative Polymer Particle Absorbent Capacities. Z before Yafter Sample centrifuge centrifuge Number capacity (g/g) capacity (g/g)Free Liquid Value (%) A 32.42^(a) 23.86^(a) 26^(a) B 32.25 23.91 26  C33.02^(a) 24.04^(a) 27^(a) ^(a)mean of two replicates.

While the preferred embodiment of the invention has been illustrated anddescribed, it will be appreciated that various changes can be madetherein without departing from the spirit and scope of the invention.

1. A superabsorbent polymer derived from a polysaccharide material, saidpolymer having a Free Liquid Value less than about 40 percent and anabsorptive capacity of at least about 20 g/g.
 2. The polymer of claim 1,wherein the polymer is a sulfated polymer.
 3. The polymer of claim 1,wherein the polymer is a crosslinked polymer.
 4. The polymer of claim 1,wherein the polymer is a regenerated polymer.
 5. A superabsorbentpolymer derived from a polysaccharide material, said polymer having aFree Liquid Value less than about 30 percent and an absorptive capacityof at least about 20 g/g.
 6. The polymer of claim 5, wherein the polymeris a sulfated polymer.
 7. The polymer of claim 5, wherein the polymer isa crosslinked polymer.
 8. The polymer of claim 5, wherein the polymer isa regenerated polymer.
 9. A regenerated, superabsorbent polymer derivedfrom a polysaccharide material, said polymer having a Free Liquid Valueless than about 40 percent and an absorptive capacity of at least about20 g/g.
 10. The polymer of claim 9 having a Free Liquid Value less thanabout 30 percent.
 11. The polymer of claim 9, wherein the polymer is asulfated polymer.
 12. The polymer of claim 9, wherein the polymer is acrosslinked polymer.
 13. A superabsorbent polymer derived from apolysaccharide material treated with sulfuric acid to render thepolysaccharide material superabsorbent, wherein the polymer has anabsorptive capacity of at least about 20 g/g.
 14. The polymer of claim13 having a Free Liquid Value less than about 40 percent.
 15. Thepolymer of claim 13 having a Free Liquid Value less than about 30percent.
 16. The polymer of claim 13, wherein the polymer is acrosslinked polymer.
 17. The polymer of claim 13, wherein the polymer isa regenerated polymer.
 18. A regenerated, superabsorbent polymer derivedfrom a polysaccharide material treated with sulfuric acid to render thepolysaccharide material superabsorbent, said polymer having a FreeLiquid Value less than about 40 percent.
 19. The polymer of claim 18having a Free Liquid Value less than about 30 percent.
 20. The polymerof claim 18 having an absorptive capacity of at least about 20 g/g. 21.The polymer of claim 18, wherein the polymer is a crosslinked polymer.22. A superabsorbent polymer derived from a sulfated polysaccharidematerial, said polymer having a Free Liquid Value less than about 40percent, wherein the polymer is not crosslinked with a crosslinkingagent.
 23. The polymer of claim 22 having a Free Liquid Value less thanabout 30 percent.
 24. The polymer of claim 22 having an absorptivecapacity of at least about 20 g/g.
 25. The polymer of claim 22, whereinthe polymer is a regenerated polymer.
 26. A method for making asuperabsorbent polymer particle, comprising sulfating a polysaccharidematerial with a sulfating agent to provide a sulfated polysaccharidematerial; dissolving the sulfated polysaccharide material in an aqueousmedium; and precipitating the sulfated polysaccharide material from theaqueous medium to provide a polymer particle.
 27. The method of claim26, wherein the sulfating agent comprises sulfuric acid.
 28. The methodof claim 26, wherein precipitating from the aqueous medium comprisesadding a non-aqueous material to the aqueous medium.
 29. The method ofclaim 26 further comprising neutralizing the sulfated cellulosic,lignocellulosic, or polysaccharide material prior to dissolving thematerial in an aqueous medium.
 30. A method for making a superabsorbentpolymer particle, comprising sulfating a polysaccharide material with asulfating agent to provide a sulfated polysaccharide material;dissolving the sulfated polysaccharide material in an aqueous medium;and regenerating the sulfated polysaccharide material from the aqueousmedium by drying to provide a polymer particle.
 31. The method of claim30, wherein the sulfating agent comprises sulfuric acid.
 32. The methodof claim 30 further comprising neutralizing the sulfated cellulosic,lignocellulosic, or polysaccharide material prior to dissolving thematerial in an aqueous medium.
 33. An absorbent composite comprising thepolymer of claim
 1. 34. The absorbent composite of claim 33 furthercomprising one or more fibrous materials.
 35. The absorbent composite ofclaim 34, wherein the fibrous material is at least one of cellulosicfibers and synthetic fibers.
 36. An absorbent article, comprising thecomposite of claim
 33. 37. The absorbent article of claim 36 furthercomprising a liquid acquisition layer.
 38. The absorbent article ofclaims 37 further comprising a liquid distribution layer.
 39. Theabsorbent article of claim 36 comprising a liquid pervious face sheetand a liquid impervious back sheet.
 40. The absorbent article of claim36, the absorbent article is at least one of an infant diaper, adultincontinence product, and a feminine care product.
 41. The polymer ofclaim 1, wherein the polysaccharide material is at least one of acellulosic material or lignocellulosic material.
 42. The polymer ofclaim 5, wherein the polysaccharide material is at least one of acellulosic material or lignocellulosic material.
 43. The polymer ofclaim 9, wherein the polysaccharide material is at least one of acellulosic material or lignocellulosic material.
 44. A The polymer ofclaim 13, wherein the polysaccharide material is at least one of acellulosic material or lignocellulosic material.
 45. The polymer ofclaim 18, wherein the polysaccharide material is at least one of acellulosic material or lignocellulosic material.
 46. The polymer ofclaim 22, wherein the polysaccharide material is at least one of acellulosic material or lignocellulosic material.
 47. The polymer ofclaim 26, wherein the polysaccharide material is at least one of acellulosic material or lignocellulosic material.
 48. The polymer ofclaim 30, wherein the polysaccharide material is at least one of acellulosic material or lignocellulosic material.